MUMBAI, India - Airports in India went on high alert yesterday following fresh attack warnings as officials said that India suspects two senior leaders of a banned Pakistani militant group orchestrated the deadly Mumbai attacks.

The alert came as Pakistani President Asif Ali Zardari promised the U.S. secretary of state that his country would take "strong action" against any elements in Pakistan involved in last week's siege.

The new alert that warned of possible airborne attacks focused on three major airports - New Delhi, Bangalore and Chennai - but security was stepped up across India. No details about the threat were released.

"This is a warning which we have received," India's air force chief, Fali Homi Major, told reporters. "We are prepared as usual."

The British Broadcasting Corp. cited unconfirmed reports from airport officials as saying late yesterday that up to six gunmen had been shot and killed at New Delhi's international airport. But Indian police told The Associated Press that there was a minor incident and no deaths. "It was not a terrorist incident. No one was killed," said police spokesman Rajan Bhagat. He gave no further details.

Heavily armed guards from India's Rapid Deployment Force manned roadblocks outside airports, while others patrolled inside airport buildings among passengers.

Several extra layers of security were set up, and some passengers had bags scanned with devices to check for explosives before entering terminals.

"Passengers have been asked to pass through six-stage security checks," said Brij Lal, a senior police official organizing security at the airport in the northern city of Lucknow.

Nirmala Sharma, a passenger who flew from New Delhi to Lucknow, said that her bags were checked half a dozen times and that she went through a metal detector three times. "Sometimes it seemed tedious, but it seems to be the need of the hour," she said.

Meanwhile, officials continued to probe the attacks.

Evidence collected in probes so far has pointed to two members of outlawed Pakistani group Lashkar-e-Taiba as masterminds in the attacks, according to two Indian government officials familiar with the matter.

The men, Zaki-ur-Rehman Lakhvi and Yusuf Muzammil, are believed to be in Pakistan, the officials said. Lakhvi was identified as the group's operations chief and Muzammil as its operations chief in Kashmir and other parts of India.

The lone surviving gunman in the assault, Ajmal Amir Kasab, 21, told police that Lakhvi recruited him for the operation, and the assailants called Muzammil on a satellite phone after hijacking an Indian vessel en route to Mumbai. During the attacks, the gunmen used mobile phones taken from hotel guests to place calls to the Pakistani city of Lahore.

The Indian officials spoke on condition of anonymity because they weren't authorized to talk publicly to discuss the details.

Police officers said that they were trying to get as much detail as possible from Kasab.

"A terrorist of this sort is never cooperative," said Deven Bharti, the head of the Mumbai crime branch. "We have to extract information."

Indian police are well known for using interrogation methods that would be regarded as torture in the West, including questioning suspects drugged with "truth serum."

Bharti provided no details on interrogation techniques, but said that "truth serum" would probably be used next week. He did not specify what drug would be used.

Kasab has so far told police that he and the other nine attackers trained for months in camps in Pakistan operated by Lashkar. *